Something that I've read is that another reason why rain is so popular and tend to be considered "broken" is because many threats are easier to fit on rain, and/or work well more with it than sand or sun, for example. Breloom, for example, is a pokémon that is easier to fit on rain. Try using it on sun, it will just compound your weakness to Fire and Ice, alongside your Chlorophyll sweeper (as there is no reason to use Breloom over the Chlorophyl sweeper). On sandstorm teams, you are forced to use sets with Poison Heal or Choice items, as Breloom is not immune to sand damage, and unlike Celebi, it does not have acess to a healing move. If you use Life Orb, your Breloom will not going to survive a lot of time (I speak this from my own experience). And it offers no defensive synergy whatsover due to its frailty. However, Breloom works well on rain because it doesn't have these problems, and can even have its Fire-weakness mitigated, and can check threats from opposing rain teams, and sand teams. Thundurus-T, on other hand, is an example of Pokémon that work well mor with rain than sand or sun. It can be, for example, a main sweeper or revenge killer for a sand team. I've remember of using its Scarf set on a sun team, and it did a good job. However, it does a much better job on rain teams because it will have access to a 100% accurate Thunder, which is clearly better than Thunderbolt. It will also protect the rain team from a great weakness that they tend to have: Electric. Sand or sun teams generally don't have the tendency to have weakness to Electric-type attacks, so Thundurus-T does not offer that much defensive synergy with them.